Voltage control system for heating loads



Jan. 17, 1967 M. E. WERTS VOLTAGE CONTROL SYSTEM FOR HEATING LOADS Filed April 29, 1963 I I I IF INVENTOR. mar/e i. Maris H15 ATTORNEY ing thermistor in a fourth leg.

United States Patent 3,299,345 VOLTAGE CONTROL SYSTEM FOR HEATING LOADS Merle E. Werts, Piqua, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware ' Filed Apr. 29, 1963, Ser. No. 276,582

Claims. (Cl. 323-22) accurate, alternating current, electrical semiconductor control system for an electrical load which is reliable, accurate, inexpensive and free of radio interference.

It is an other object of my invention to provide a suitable, accurate, electrical semiconductor control system for an electrical load which controls by either conducting or failing to conduct for one or more substantially complete half waves.

It is another object of my invention to provide a suitable accurate alternating current, electrical semiconductor control system for an electrical load which applies a peak voltage triggering signal to the semiconductor substantially at the time the voltage applied to it passes through zero.

It is another object of my invention to provide a suitable, accurate, electrical semiconductor control system for an electrical load which applies a peak voltage triggering signal to the semiconductor substantially at the time the voltage applied to it passes through zero and which controls the conduction by controlling the amplification of the peak voltage.

These and other objects are attained in the form shown in the drawings in which a sheathed, tubular, electrical resistance type of surface heater is provided at its center point with a spring mounted contact device embodying a thermistor adapted to contact the bottom of a pan or utensil resting on the heater. This heater is connected in series witha silicon controlled rectifier across an alternating current supply source. According to my invention, at the beginning of a half wave substantially at or slightly before the voltage applied to the rectifier passes through zero, there is applied to the gate of the silicon controlled rectifier a voltage peak. This is obtained through the use of a first step-down transformer energized by the alternating current supply source which transmits energy at a reduced voltage through a transistor and a diode to a second step-up transformer which, in turn, transmits higher voltage energy through a second diode to the gate of the rectifier substantially at or slightly before the time the alternating current sine wave applied to the rectifier passes through zero. The wave shape of the energy applied to the gate establishes a peak voltage at the beginning of each sine wave followed by reducing or lower voltages.

A second winding upon first transformer supplies a Wheatstone bridge circuit which includes a sensing thermistor previously mentioned, a potentiometer in the leg opposite the sensing thermistor, an ambient temperature thermistor compensator in a third leg and an anticipat- One terminal of the bridge is connected to the base of the transistor while 3,299,345 Patented Jan. 17, 1967 the second terminal is connected to the emitter. When the sensing thermistor is below the temperature selected by the potentiometer, the Wheatstone bridge is unbalanced and supplies a current to the base of the transistor thereby sufficiently amplifying the current in the transformer circuitry so as to supply sufiicient voltage and current to the gate of the rectifier to cause the rectifier to become highly conductive for all of each similar half cycle until the sensing thermistor substantially reaches the temperature selected by the adjustment of the potentiometer. The anticipating thermistor reduces the current flow to the gate of the rectifier sufiiciently to prevent any substantial amount of overshooting as the selected temperature is reached.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

FIGURE 1 is a top view of the surface heater of an electric range embodying one form of my invention;

FIGURE 2 is a wiring diagram which may be applied to FIGURE 1 embodying one form of my invention; and

FIGURE 3 is a voltage-time diagram showing the voltage pulse applied to the gate 'of the rectifier coordinated with the alternating voltage sine wave received directly from the supply conductors.

Referring now to the drawings and more particularly to FIGURE 1, there is shown an electric surface heater 20 for the top 22 of an electric range. The heater 20 is supported from the top 22 by a three-armed frame 24 which also supports the spring mounted sensing thermistor 26 in the center of the heater 20. The heater 20 is of the spiral, sheathed, tubular type adapted to support pans and other cooking utensils. The sensing thermistor 26 'is spring mounted to be held against the bottom of the pan or utensil which rests upon the surface heater 20.

One terminal of the surface heater 20 is connected by the conductor 28 to'the supply conductor L The other terminal of the heater 20 is connected by the conductor 30 to a .02 ohm anticipating feedback heater 32 which, in turn, is connected through the conductor 34 to the anode of a silicon controlled rectifier 36. This rectifier, for example, maybe type 2N688. The cathode of the rectifier 36 is connected by the conductor 38 to the supply conductor L The alternating current supply conductors L and L preferably supply standard alternating current at a voltage of about 235 volts. This is represented by the curve 40 in FIGURE 3 illustrating a sine wave providing alternate half cycles on opposite sides of the zero voltage line 42. Heretofore, control devices have been provided which cause the silicon controlled rectifier to become conducting for only the latter portions of each alternate half cycle. This is done by causing the silicon controlled rectifier to conduct at some point intermediate the two zero points in each alternate half cycle. This starting point is varied so as to secure the desired average amount of. current fiow for any desired condition. All this affects modulation in a very uniform manner. But it has the very objectionable characteristic of creating an objectionable amount of radio interference. For this reason, such a control cannot be used in many localities.

According to my invention, at the beginning of each positive half cycle just prior to the time the voltage of each positive half cycle passes through zero, as illustrated in FIGURE 3, I apply a peak voltage 47 to the gate 44 of the rectifier 36, as indicated by the upper curve or line 46 shown in FIGURE 3, directly above the voltage curve 40 and coordinated directly with it. The point immediately above each zero point is illustrated by the vertical dotted line 48. The voltage peak 47 illustrated by the curve 46 in FIGURE 3 is delivered to the gate 44 from the secondary of the step-up transformer 50 through a diode 52. The transformer 50, for example, may have its primary to secondary turns in the ratio of one to eight. This transformer may be a Staco 9407 transformer manufactured by the Standard Electrical Products Company of Dayton, Ohio. The diode 52 may be a type IN2069. The second terminal of the secondary winding of the transformer 50 is connected by the conductor 54 to the conductor 38 which connects to the cathode of the silicon controlled rectifier 36.

One terminal of the primary winding of the transformer 50 is connected by the conductor 56 to one terminal of the secondary winding 58 of a step-down transformer 60 which preferably is of the same manufacture and type, namely Staco 9407. The transformer 60 has its primary winding 62 with one terminal connected to the supply conductor L and the other terminal connected through a neutral supply conductor N to provide a 117 volt supply. This transformer has one secondary winding 64 providing a six-volt A.C. output while the other secondary winding 58 provides a nine-volt A.C. output. The second terminal of the secondary winding 58 is connected through a conudctor 66 to the emitter electrode of a current controlling transistor 68. This transistor 68, for example, may be a type 2N1305. The collector electrode of this transistor 68 is connected through the diode 70 to the second terminal of the primary Winding of the transformer 50. The diode 70 may, for example, be a type 1N2069. This arrangement provides at the proper time the peak voltage 47 and current at the gate 44 of the rectifier 36 to obtain the substantially radio interference free operation desired by producing the type of voltage curve substantially as illustrated by the reference character 46 in FIGURE 3 with the peak voltage occurring at substantially the same time as the voltage curve 40 crosses the zero for each positive half wave of the alternating voltage sine wave curve 40 illustrated in FIGURE 3 which is impressed across the silicon controlled rectifier 36. This particular synchronization of the peak voltage 47 at the gate 44 with the alternating current sine wave is provided by its supply circuit which has just been described. This apparently results from the application of the rectified half wave current to the input winding of the transformer 50 and the nonideal transformer characteristics which account for the wave shape 46 with the initial peak 47 while the rectifier 52 prevents reverse conduction thereby eliminating the sinusoidal portion 51 of the waveform 46 and returns it to the zero voltage line 53. As long as the current supplied from the secondary winding 58 as controlled by the current controlling transistor 68 is not great enough, the peak voltage 46 as illustrated in FIGURE 3 applied to the gate 44 will not be sufficient to cause the silicon controlled rectifier 36 to conduct. The silicon controlled rectifier 36 is caused to conduct by increasing the base to emitter voltage upon the cu-rrent controlling transistor 68. This is done through a Wheatstone bridge circuit 72 which is fed from the secondary winding 64 through the conductors 74 and 76. The conductor 76 connects to the input terminal 80 connecting with the lower two legs of the bridge circuit, one of which includes the sensing thermistor 26 and the other of which is the adjustable potentiometer 78. The Wheatstone bridge circuit 72 has input terminals 80 and 82 which connect to the supply conductors 76 and 74 and output terminals 84 and 86 which connect respectively through the conductor 88 and the conductor 66 to the emitter electrode of the transistor 68 and through the con- I minal 84 is :a resistance 92 having a value of 470 ohms, While connected between the input terminal 82 and the output terminal 86 is an identical resistance 94 of 470 ohms. Connected in shunt with the resistance 92 is an ambient temperature compensating thermistor 95 having a resistance of 10,000 ohms at 77 P. which reduces to about 40 ohms at 500 F. Connected in shunt with the resistance 94 is the anticipating thermistor 96 also having a resistance of 10,000 ohms at 77 P. which reduces to 40 ohms at 500 F. This anticipating thermistor is associated in heat transfer relation with the'strip heater 32 which has a value of 7 6 ohm. The heat transfer between the anticipating feed back heater 32 and the anticipating thermistor 96 is such that eleven amps of current will bring the thermistor from a resistance of 10,000 ohms to a resistance of 1000 ohms in ten seconds.

The pan temperature selected to be attained and maintained is provided by suitably adjusting the potentiometer 78 to a resistance which will be equal to the resistance of the thermistor 26 at the desired or selected temperature. Therefore, in starting a cooking operation, the potentiometer 78 is adjusted to the proper resistance in accordance with a suitable temperature scale. Since the pan or utensil and the thermistor 26 will start at a low temperature, such as room temperature, it will have a much higher resistance than the potentiometer 78, thereby applying a voltage and current through the output terminal 84 and the conductors 88 and 66 to the emitter of the current controlling transistor 68 through the base and through the conductor to the terminal 86. This causes the transistor '68 to conduct readily and permit sufficient current in the transformer circuit to raise the voltage peak applied to the gate 44 high enough to cause the rectifier 36 to conduct current at a high rate to provide current fiow through the heater 20 at a corresponding rate so as to heat the pan or utensil resting upon the heater 20. p

As the pan or utensil rises in temperature, the resistance of the thermistor 26 will decline, thereby reducing the voltage and current flow to the base of the current controlling transistor 68. The ambient temperature compensating thermistor compensates for the elfect of environment temperature on the anticipating thermistor 96. During the heating period, the small heater 32 slowly applies heat to the anticipating thermistor 96. This has the effect of gradually reducing the 'voltage and current applied through the conductor 90 to the base of the transistor 68 so that, in combination with the sensing thermistor 26, overshooting of the temperature by the residual heat in the heater 20 will be prevented. This arrangement causes the voltage applied and the current flow through the conductor to the base of the current controlling transistor 68 to come to zero slightly beforethe pan or utensil reaches the desired temperature. This reduces the voltage peak applied to the gate 44 sufliciently that the silicon controlled rectifier 36 will cease conducting. The heating of the heater 20 as well as the heater 32 will therefore be stopped. However, any slight fall in temperature of the pan or utensil and the thermistor 26 will again increase the resistance of the sensing thermistor 26 to cause a current flow through the current controlling transistor 68, as previously described, to cause conduction and raising of the voltage peak 47 to again apply sufficient peak voltage to the gate 44 to cause the silicon controlled rectifier 36 to again conduct and cause current flow through the heater 20 to maintain the desired, selected pan or utensil temperature. The silicon controlled rectifier 36 then is eifectively turned on or turned off by increasing or lowering the conduction of the current controlling transistor 68 in accordance primarily with the temperature of the thermistor 26. Through all this increasing and reducing of the conduction by the current controlling transistor 68, the peak voltage curve 46 remains substantially in synchronization with the alternating current curve 40 so that radio interference is substantially prevented atall times. Only the value of the peak voltage 46 is changed without changing the relation of the peak 47 of this voltage curve 46 to the alternating current sine wave curve 40.

While the embodiment of the present invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In combination, an electrical heat dissipating load, an alternating current energy supply electrically connected to supply'energy to said load, a voltage responsive semiconductor control means having electrodes connected in series with said load for controlling the supply of energy from said supply to said load, preponderantly inductive circuit means for repeatedly applying to said control means a voltage peak substantially at the time in the application of the alternating voltage sine wave to said control means when the voltage passes through zero comprising a first transformer having input terminal means connected to said energy supply and output terminal means, a second transformer having input and output terminal means, means comprising first conductor means and a current control means and a rectifier connecting the output terminal means of said first transformer with the input terminal means of said second transformer, and second conductor means connecting the output terminal means of said second transformer with two electrodes of said semiconductor control means for controlling the energy supply to said load in substantially complete half cycles in response to said current control means.

2. A combination as defined in claim 1 in which a temperature responsive resistance is located in heat transfer relation with said load and electrically connecting said supply and said current control means for controlling the energization of said load, said temperature responsive resistance being provided with means for controlling said current control means.

3. A combination as defined in claim 1 in which a feedback element is energized by the application of alternating current energy to said load and said current control means comprises a circuit connected to said supply provided with means responsive to the energization of said feedback element.

4. A combination as defined in claim 1 in which a temperature responsive resistance is located in heat transfer relation With said load and electrically connecting said supply and said current control means for controlling the energization of said load, said temperature responsive resistance being provided with means for controlling said current control means, said current control means also comprising a variable resistance for varying the effect of said temperature responsive resistance in the control of the energization of said load.

5. A combination as defined in claim 1 in which the semiconductor control means is a silicon controlled rectifier having a gate, and said second conductor means includes a rectifier and connects to said gate.

References Cited by the Examiner UNITED STATES PATENTS 2,998,547 8/ 1961 Berman 315200 3,051,813 8/1962 Busch et a1 219-489 3,070,739 12/1962 Hansen et a1 32l47 3,175,076 3/1965 Fox et a1. 219494 3,175,077 3/1965 Fox et a1. 2l9494 3,204,113 8/1965 Snygg 323-22 X JOHN F. COUCH, Primary Examiner.

W. E. RAY, Assistant Examiner. 

1. IN COMBINATION, AN ELECTRICAL HEAT DISSIPATING LOAD, AN ALTERNATING CURRENT ENERGY SUPPLY ELECTRICALLY CONNECTED TO SUPPLY ENERGY TO SAID LOAD, A VOLTAGE RESPONSIVE SEMICONDUCTOR CONTROL MEANS HAVING ELECTRODES CONNECTED IN SERIES WITH SAID LOAD FOR CONTROLLING THE SUPPLY OF ENERGY FROM SAID SUPPLY TO SAID LOAD, PREPONDERANTLY INDUCTIVE CIRCUIT MEANS FOR REPEATEDLY APPLYING TO SAID CONTROL MEANS A VOLTAGE PEAK SUBSTANTIALLY AT THE TIME IN THE APPLICATION OF THE ALTERNATING VOLTAGE SINE WAVE TO SAID CONTROL MEANS WHEN THE VOLTAGE PASSES THROUGH ZERO COMPRISING A FIRST TRANSFORMER HAVING INPUT TERMINAL MEANS CONNECTED TO SAID ENERGY SUPPLY AND OUTPUT TERMINAL MEANS, A SECOND TRANSFORMER HAVING INPUT AND OUTPUT TERMINAL MEANS, MEANS COMPRISING FIRST CONDUCTOR MEANS AND A CURRENT CONTROL MEANS AND A RECTIFIER CONNECTING THE OUTPUT TERMINAL MEANS OF SAID FIRST TRANSFORMER WITH THE INPUT TERMINAL MEANS OF SAID SECOND TRANSFORMER, AND SECOND CONDUCTOR MEANS CONNECTING THE OUTPUT TERMINAL MEANS OF SAID SECOND TRANSFORMER WITH TWO ELECTRODES OF SAID SEMICONDUCTOR CONTROL MEANS FOR CONTROLLING THE ENERGY SUPPLY TO SAID LOAD IN SUBSTANTIALLY COMPLETE HALF CYCLES IN RESPONSE TO SAID CURRENT CONTROL MEANS. 